CHT's revisited

[dkozak12]

O-320-A2B 160hp 1020SMOH. Rear mounted oil cooler. Stock exhaust. Alcor CHT with spark plug ring on #4. At cruise 380-420, can see 470 on a long climb. New baffle seals two months ago and no change. Temps stay the same whether it's -15 in the winter or 80 in the summer. The motor flies 200-275 hours a year.

For the other guys who have delt with high CHT temps, what has been your order of trouble shooting? Seems like there are many options and half of the folks I talk to just say rip out the gauge and run it, it's a cub. Other's say the spark plug ring is useless and read high. Some say drop $2,000 and install an engine analyzer. Seems like guys have luck with baffle ramps, are they worth it. Would an aftermarket exhaust like the AD hotrod help bring the temps down? Anyone seen this after installing a new exhaust system? Should the carb jet be checked? Replace with a bayonet type CHT probe and see what happens? Is this even a serious enough issue to start throwing $$$ at or just keep running it?

For the smart engine folks out there, what are the first indications of high temp damage in a O-320? Exhaust valves?

THANKS FOR THE HELP!

Dan

 

[Tim]

Start by getting the bayonet type CHT probes and go from there

 

[stewartb]

When I had an 0-320B2B I had marginally high CHTs. The hot rod muffler made a 20* difference compared to the new Atlee HD muffler it replaced.

 

[PerryB]

When I had an 0-320B2B I had marginally high CHTs. The hot rod muffler made a 20* difference compared to the new Atlee HD muffler it replaced.

Same here. I did not have temp issues to begin with, but did experience the same 20* temp. reduction.
** Do what Tim says first and get a bayonet style probe so you're comparing apples/apples.

 

[Aviator]

Bayonets are better. I'd check all cylinders for spread. Ramps are effective if front cylinders run much cooler.

 

[DENNY]

Dan
Lots to deal with here. First and number one is a single cylinder CHT is about as useful at tits on a boar hog!!!!! What are the other cylinders doing?? Is number 4 the hottest or is it number 3??? Could a bit of baffling on the front of 1 and 2 drop the rear by 50 degrees? The big issue is the exhaust valve guides and the higher the temp the sooner they fail. First thing is to get a 4 cylinder EGT/CHT than Use aluminized tape to adjust airflow and lay it under cowl hinges to keep things tight. Do not throw good money into anything until you have a good baseline temp to start with!!!!!!!!!!!!
DENNY

 

[skywagon8a]

.......Anyone seen this after installing a new exhaust system? .....

Is this what you did? How long has this high temp been going on? Always? Or just started? Has your thermocouple lead been rubbing and chafed through grounding itself causing an erroneous reading? The spark plug thermocouple rings/leads do get damaged over time.

 

[Steve Pierce]

The bayonet probe can be 50 degrees lower than the spark plug gasket probe.

 

[aktango58]

It was asked by Skywagon: When did these temps appear?

Is this a situation you just put the temp deal on and went "oh shoot!", or has it been running much lower and jumped up?

Before you get going on jets in the carb, can you push in the mixture in and make it run rich? If so, that is probably not an issue.


Has the temp gauge been checked for accuracy? Take it off, take it home and test it in hot water, or in the oven with a good kitchen thermometer.

These are things my non-mechanic brain thinks of before I take it into the shop for spending big dollars.

One other quick check: with the plane running can you spray starter fluid onto the #4 intake manifold and get an rpm change? Could have a leak.

 

[dkozak12]

EXCELLENT! Thanks for the knowledge and the help! The bayonet temp probe is on order and will get it put on next week, curious to see if there is a change.

To answer a few questions: these temps have been consistent and didn't just appear. This is my second year with the airplane and it's been a smooth motor with no change in personality. I put 270 of the 1020hours on the motor and believe it has probably run at these temps since overhauled in 2006. Oil analysis from the last 270hrs shows chrome and iron just a touch above "average" but doesn't seem of much concern. I tried the starter fluid trick and no RPM increase on any cylinder, neat trick though, I will definitely keep that one in my hip pocket to find leaks. Like DENNY said, it all seems point less to measure only one CHT on one cylinder without EGT, fuel flow, etc. Seems like the logical choice would be a basic engine analyzer.

 

[RaisedByWolves]

I recently put the cgr-30 in and now I have high oil temps, and CHT's. I've tightened the baffles up, and that helped, now I'm working on the cowl, trying to get more airflow. I made Ramps on the front of the front cyl's and that helped on 1&3 (360 or so) and 2&4 run about 390. Trying to get those down. Oil has been hot too. Hopefully the increase in airflow out the cowl will help.

 

[fobjob]

Search on CHT's....
when you measure CHT's, you really need to note OAT, use the same power setting in RPM, and use the same IAS. Otherwise, you are all over the place.
When I set my rpm to be 2450, my IAS to 100 mph, "figure of merit" (a constant) FOM + OAT(deg F) = CHT.... a good FOM is 300, a bad FOM is 390.....once you know your FOM,
you can judge your cooling system performance with some semblance of accuracy.

 

[Tim]

I know a guy ( on here ) that put Husky vents on each side of his PA-12 lower cowl. They are about 12" by 6". I think he said they reduced his CHTs 15 to 20 deg. If he sees this thread he might give his opinion

 

[Poor Joe]

I have a narrow deck 0320 on a 12 that was overhauled by Penn Yan. When it came back I decided to install a 4 probe JPI egt/cht. Went through the normal break in for about 20 hours but could not climb more tha about 1500 feet before the#4 cylinder went through 450 which is where I leveled off until things got back to 390 and would try do it again with the same results. Previous to this during other modifications the engine mount, cowling etc went from long to short. With the long mount I could slide 3 fingers in back by the boot cowl and barely 1 on the new short mount cowl so I always felt it was reduced airflow being the problem. First we made sure the JPI was right then re did the baffling, looked for any possible air leaks and resealed around the air filter to block any air from getting in below the engine. All that produced only a very minor improvement. Next we installed a lip on the lower cowl like that on the tri pacer. This helped a good deal and at least kept the #4 cylinder at around 415 to 420 in a climb. No more step climb. About a month later I hit my head walking under a husky and while cursing noticed the vents. I don't recall seeing a temp above 380 in a climb since we installed them. I will put some pictures together and get them to you if you like when I get near the plane.

 

[stewartb]

Making new cowl doors to increase the gap is pretty simple.

Any carbureted engine should have 125-150* of leaning authority in level cruise. If not the fuel flow may be the cause of high temps. I've had a little practice with that issue. You can chase lots of things with little success. The key is to correctly identify the problem. One comment that caught my eye is that the OP's temps are the same in winter and summer. Mine used to be before I had enough fuel. Once the fuel flow was increased the CHTs varied with the OAT. Maybe that's a clue.

One more thing. IF the takeoff fuel flows are borderline low in summer you're asking for trouble come winter time. Alaskan planes need to be set up for the coldest temps you'll fly in. The mixture knob will allow you to lean for warmer conditions. A local Alaskan pilot/mechanic friend had to point out the obvious to me. Sometimes the simple stuff is overlooked.

 

[SC3CM]

About a month later I hit my head walking under a husky and while cursing noticed the vents. I don't recall seeing a temp above 380 in a climb since we installed them. I will put some pictures together and get them to you if you like when I get near the plane.

If you take the time to post the pics I'll have a look.

 

[JimC]

When doing engine cooling calculations, I suggest using the P&W face rise equation rather than accepting at face value (pun intended) the Lycoming assumption of 150 degree face rise.

 

[fobjob]

The real measure of cooling performance is to get on ebay and get a Magnehelic differential pressure gauge of 0-5 or so inches of water pressure. Measure the pressure in the suction side of the cowl with respect to static. You want the pressure to be zero. Anything higher is air bypassing the baffles, and/or not enough exhaust air capability. Lycoming wants 4 inches of air pressure differential across the engine, ram air pressure at 100 mph is 4.2 inches. Do the math. When I started probing the mystery of why my engine was so hot, my suction side pressure was +2.6 inches. 4.2-2.6= 1.6 inches across the engine.....

 

[Poor Joe]

Stewart although I am certainly far from being an expert you are definitely right to look their first. I had forgotten about this and the local shop here did start there by adding what I was told was the next size up and the largest jet available for that carb. On takeoff the flow was approx 13.8 gal/hr and was part of the initial work done that I mentioned before but for me the biggest change came with the fins. I wish my memory served me better and am trying to find the notebook where most of this stuff was jotted down.

 

[stewartb]

PJ,

I don't presume to know what's wrong with all planes that exhibit higher than desired CHTs, just trying to recount my own circumstances in case they mean something useful to others.

SB

 

[JimC]

El cheapo way to measure the pressure difference across the face is a length of aquarium tubing with one end in the upper plenum and the other in the lower. Bring the tube into the cabin, with the clear tube containing a few inches of water plus food coloring, wrapped in a U-shape around a yardstick. You can make it less sensitive to pressure surges by making brass piccolo tubes to insert in each end.

Air mass flow required vs hp and air mass flow vs pressure difference curves for each engine are available from Lycoming.

Dynamic ram recovery efficiency depends upon the inlet lip to inlet throat area ratio and on the shape of the diffusion ramps, but is typically on the order of 50 to 75%. You'll typically find that the outlet opening can't be made large enough to avoid the need for step climbing when Lycomings are climbing out below 79 mph.

 

[cplne]

Would love to see a pic of those Husky louvers and what your seaplane lip looks like. I’m going to start laying one up this week

 

[mvivion]

Would love to see a pic of those Husky louvers and what your seaplane lip looks like. I’m going to start laying one up this week

The louvers are obvious in this photo, the “seaplane lip” not so obvious, but you can see it.

MTV


Better view of lip:

 

[cplne]

Cool. No pun intended